Research and development efforts are being made to utilize display devices as the display means for various electronic office systems. Using display devices for such purposes requires development of larger liquid crystal cells of the multidivisional designs. The most typical of the various types of display devices presently in use are those which utilize twisted-nematic liquid crystals. As well known in the art, however, display devices of this type are rather inferior in contrast and view-angle characteristics to display devices using cathode-ray tubes. The view-angle characteristics of display devices using liquid crystals tend to still deteriorate when the liquid crystal cells are larger sized for use in electronic office systems because of the fact that the display screens using such large-sized cells require wider angles of view. The deterioration in the view-angle characteristics apparently results in degraded qualities of the display. Division of a liquid crystal cell into increased numbers of segments as in display devices for use in electronic office systems causes significant reduction in the duty ratios operable for driving the electrodes and this, in turn, results in further deterioration of the qualities of the pictures to be produced on the screen.
Display devices using twisted-nematic liquid crystal cells thus present problems which result from the two kinds of performance characteristics, viz., the view angle and contrast characteristics of the liquid crystal cells. As well known in the art, these performance characteristics of liquid crystal cells depend largely on the properties and accordingly the types of the liquid crystals used. Thus, a liquid crystal having a relatively small birefringence index will suit the purpose of achieving improved view-angle characteristics while a liquid crystal having a relatively large birefringence index will be preferred for achieving improved contrast characteristics. There are presently known no such liquid crystals that meet both of these two mutually conflicting requirements. A compromise has therefore been accepted in display devices presently available on a commercial basis. According to this compromise, a liquid crystal having a relatively large birefringence index is used to place a priority on the contrast characteristics of the display device rather than on the view-angle characteristics which are prescribed to provide the optimum contrast solely in particular directions which are likely to be most frequently used.
Typical of the liquid crystals presently used for providing high contrast characteristics are those which have birefringence indices ranging from 0.13 to 0.15, view angles ranging from 10 degrees to 35 degrees and a contrast ratio of 8:1. Examples of the liquid crystals used for providing high view-angle characteristics include those which have birefringence indices ranging from 0.08 to 0.10, view angles ranging from 10 degrees to 40 degrees and a contrast ratio of 6:1. For display devices of the multidivisional types, the liquid crystals providing high contrast characteristics are used exclusively.
Display devices for use in electronic office systems in general are broken down to those forming vertically elongated screens and those forming horizontally elongated screens. Where display devices are used to provide vertically elongated screens, the viewers are required to be positioned in front of the display screens to view the total areas of the screens and, thus, the rather inferior view-angle characteristics inherent in display devices become the important problem. As well known in the art, the view angles to provide the optimum range of contrast by multidivisional display devices using liquid crystals providing high contrast characteristics range from 10 degrees to 35 degrees from the directions normal to the display screens to preferential directions of viewing. display means for an electronic office system, it is thus important to improve the view-angle characteristics of the display device as a whole to cope with the enlarged visual angles with respect to the display screens. Where a vertically elongated display screen is used for the device, it is required that the range of the view angles to provide the optimum range of contrast be shifted to a range containing zero degrees to the directions normal to the display screen or, in other words, the directions of viewing to provide the optimum contrast be made identical with the directions normal to the display screen.
There are presently two major approaches to realizing the arrangement in which the directions of viewing with the optimum contrast are to be made identical with the directions normal to the display screen.
One approach is to have recourse to using higher voltages to be applied between the common and segment electrodes of the display device for thereby achieving the optimum contrast when the display screen is viewed particularly in the directions normal to the screen. Adoption of this expedient however results in darkening of the images throughout the display area and further presents a problem that the range of the view angles to provide the optimum range of contrast tends to be narrower than the ordinarily used range of 10 degrees to 35 degrees. The use of higher voltages will also result in problems such as the increase in the power consumption and the accelerated deterioration of the liquid crystal cell.
The other approach is to place a Fresnel lens as a prism on top of the display structure while maintaining the view-angle characteristics of the device unchanged. With the prism thus positioned on the display structure, the display patterns with the optimum contrast achievable by the range of the view angles of the display screen are oriented into directions normal to the display screen. This approach might however cause deformation of the images on the screen and critical reduction of the contrast due to the scattering of the display light through the prism and to admission of noise light into the display screen, if the Fresnel lens used fails to be correctly positioned with respect to the matrix pattern of the electrodes. Extreme difficulties are experienced in achieving proper registration between the Fresnel lens and the electrode pattern and, for this reason, the second approach to providing directions of viewing with the optimum contrast in the directions normal to the display screen is a far cry to practical use.
The present invention contemplates elimination of these and other problems encountered in prior-art display devices using multidivisional, large-sized liquid crystal cells.
It is, accordingly, an important object of the present invention to provide a display device using an improved multidivisional, large-sized liquid crystal cell providing satisfactory view angle and contrast characteristics.
It is another important object of the present invention to provide a display device using an improved multidivisional, large-sized liquid crystal cell having excellent view-angle characteristics and adapted to achieve optimum contrast in the directions normal to the display screen.